Pedogenesis (from the Greek pedo-, or pedon, meaning 'soil, earth,' and genesis, meaning 'origin, birth') (also termed soil development, soil evolution, soil formation, and soil genesis) is the process of soil formation as regulated by the effects of place, environment, and history. Biogeochemical processes act to both create and destroy order (anisotropy) within soils. These alterations lead to the development of layers, termed soil horizons, distinguished by differences in color, structure, texture, and chemistry. These features occur in patterns of soil type distribution, forming in response to differences in soil forming factors.
Pedogenesis is studied as a branch of pedology, the study of soil in its natural environment. Other branches of pedology are the study of soil morphology, and soil classification. The study of pedogenesis is important to understanding soil distribution patterns in current (soil geography) and past (paleopedology) geologic periods.
Soil develops through a series of changes. The starting point is weathering of freshly accumulated parent material. Primitive microbes feed on simple compounds (nutrients) released by weathering, and produce acids which contribute to weathering. They also leave behind organic residues.
New soils increase in depth by a combination of weathering, and further deposition. An estimated 1/10 mm per year rate of soil production from weathering fits observations rates. New soils can also deepen from dust deposition. Gradually soil is able to support higher forms of plants and animals, starting with pioneer species, and proceeding to more complex plant and animal communities. Soils deepen with accumulation of humus primarily due to the activities of higher plants. Topsoils deepen through soil mixing. As soils mature, they develop layers as organic matter accumulates and leaching takes place. This development of layers is the beginning of the soil profile.
Factors of soil formation
Russian geologist Vasily Dokuchaev (1889), commonly regarded as the father of pedology, determined in 1883 that soil formation occurs over time under the influence of climate, vegetation, topography, and parent material. He demonstrated this in 1898 using the soil forming equation:
- soil = f(cl, o, p) tr
(where cl or c = climate, o = organisms, p = biological processes) tr = relative time (young, mature, old)
- S = f(cl, o, r, p, t, …)
- S soil formation
- cl (sometimes c) climate
- o organisms (soil microbiology, soil mesofauna, soil biology)
- r relief
- p parent material
- t time
Published in 1941, Jenny's state equation in Factors of Soil Formation differs from the Vasily Dokuchaev equation, treating time (t) as a factor, adding topographic relief (r), and pointedly leaving the ellipsis "open" for more factors (state variables) to be added as our understanding becomes more refined.
There are two principal methods that the state equation may be solved: first in a theoretical or conceptual manner by logical deductions from certain premises, and second empirically by experimentation or field observation. The empirical method is still mostly employed today, and soil formation can be defined by varying a single factor and keeping the other factors constant. This led to the development of empirical models to describe pedogenesis, such as climofunctions, biofunctions, topofunctions, lithofunctions, and chronofunctions. Since Hans Jenny published his formulation in 1941, it has been used by innumerable soil surveyors all over the world as a qualitative list for understanding the factors that may be important for producing the soil pattern within a region.
Heat and moisture affect rates of biological activity and chemical reactions. Seasonal patterns of heat flux, water content and water movement influence the depth and pattern of removal (elluviation) and accumulation (illuviation) of soluble and colloidal constituents in soil. Climatic extremes, such as ice and wind, can cause physical weathering, soil erosion as well as deposition and accumulation of soil parent material. Stable, humid climates cause deep soil development. Soils are more developed in areas with higher rainfall and more warmth. Soils can develop faster in warmer climates. The rate of chemical weathering can nearly double for each 10 degrees Celsius increase in temperature. Climate also affects which organisms are present, affecting the soil chemically and physically (movement of roots). Soils with similar climate histories tend to produce similar soils.
Each soil has a unique combination of microbial, plant, animal and human influences acting upon it. Microorganisms are particularly influential in the mineral transformations critical to the soil forming process. Additionally, some bacteria can fix atmospheric nitrogen and some fungi are efficient at extracting deep soil phosphorus and increasing soil carbon levels in the form of glomalin. Plants hold soil against erosion, and accumulated plant material build soil humus levels. Plant root exudation supports microbial activity. Animals serve to decompose plant materials and mix soil through bioturbation.
The influence of man, and by association, fire, are state factors placed within the organisms state factor. Man can import, or extract, nutrients and energy in ways that dramatically change soil formation. Accelerated soil erosion due to overgrazing, and Pre-Columbian terraforming the Amazon basin resulting in Terra Preta are two examples of the effects of man's management.
The organisms living in and on the soil form distinct soil types. Coniferous forests have acidic leaf litter that form soils classed as inceptisols. Mixed or deciduous forests leave a deeper layer of humus and tend to form soils classed as alfisols. Prairies have very high humus accumulation, which, along with bioturbation can create a dark, thick A horizon characteristic of mollisols.
Soil biology affects mineral weathering, and helps determine to what degree minerals leach from or accumulate in the soil. Biologically mediated chemical weathering can create striking differences in color stratification.
Distinct ecosystems produce distinct soils, sometimes in easily observable ways. For example, three species of land snails in the genus Euchondrus in the Negev desert are noted for eating lichens growing under the surface limestone rocks and slabs (endolithic lichens). They disrupt and eat the limestone. Their grazing results in the weathering of the stones, and the subsequent formation of soil. They have a significant effect on the region: the total population of snails is estimated to process between 0.7 and 1.1 metric ton per hectare per year of limestone in the Negev desert.
The effects of ancient ecosystems are not as easily observed, and this challenges the understanding of soil formation. For example, the chernozems of the North American tallgrass prairie have a humus fraction nearly half of which is charcoal. This outcome was not anticipated because the antecedent prairie fire ecology capable of producing these distinct deep rich black soils is not easily observed.
The location of a soil on a landscape can affect how the climatic processes impact it. The geomorphic effects of relief and topography determine how soil is moved, distributed and retained within a watershed and across the landscape. Soil material is carried to lower elevations by water and with gravity. Bottom lands and low lands that retain and accumulate deposited soil will be deeper and richer with organic matter than their comparable uplands.
Soils at the bottom of a hill will get more water than soils on the slopes, and soils on the slopes that face the sun's path will be drier than soils on slopes that do not. Topography determines exposure to weather, fire, and other forces of man and nature. Mineral accumulations, plant nutrients, type of vegetation, vegetation growth, erosion, and water drainage are dependent on topographic relief.
Recurring patterns of topography result in toposequences or soil catenas. These patterns emerge from topographic differences in erosion, deposition, fertility, soil moisture, plant cover, other soil biology, fire-history, and exposure to the elements. These same differences are important to understanding natural history and when managing the land resource.
The primary material from which soil is formed is called parent material. Soil parent material could be bedrock, organic material, an old soil surface, or a deposit from water, wind, glaciers, volcanoes, or material moving down a slope.
All of the above factors assert themselves over time, often thousands of years. Soil profiles continually change from weakly developed to well developed over time. Chronosequences used in soil studies consist of sites that have developed over different periods of time with relatively small differences in other soil-forming factors. Such groups of sites are used to assess the influence of time as a factor in pedogenesis.
Paleosols are soils formed during previous soil forming conditions.
Soil forming processes
Soils develop from parent material by various weathering processes. Organic matter accumulation, decomposition, and humification are as critically important to soil formation as weathering. The zone of humification and weathering is termed the solum.
Soil mixing (pedoturbation) is often an important factor in soil formation. Pedoturbation includes churning clays, cryoturbation, and bioturbation. Types of bioturbation include faunal pedoturbation (animal burrowing), floral pedoturbation (root growth, tree-uprootings), and fungal pedoturbation (mycelia growth). Pedoturbation transforms soils through destratification, mixing, and sorting, as well as creating preferential flow paths for soil gas and infiltrating water. The zone of active bioturbation is termed the soil biomantle.
Soil moisture content and water flow through the soil profile support leaching of soluble constituents, and eluviation. Eluviation is the translocation of colloid material, such as organic matter, clay and other mineral compounds. Transported constituents are deposited due to differences in soil moisture and soil chemistry, especially soil pH and redox potential. The interplay of removal and deposition results in contrasting soil horizons.
A variety of mechanisms contribute to soil formation, including siltation, erosion, overpressure and lake bed succession. A specific example of the evolution of soils in prehistoric lake beds is in the Makgadikgadi Pans of the Kalahari Desert, where change in an ancient river course led to millennia of salinity buildup and formation of calcretes and silcretes.
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- Scalenghe, R., Territo, C., Petit, S., Terribile, F., Righi, D. (2016). "The role of pedogenic overprinting in the obliteration of parent material in some polygenetic landscapes of Sicily (Italy)". Geoderma Regional. 7: 49–58. doi:10.1016/j.geodrs.2016.01.003.CS1 maint: multiple names: authors list (link)
- Wilkinson, M.T., Humpreys, G.S. (2005). "Exploring pedogenesis via nuclide-based soil production rates and OSL-based bioturbation rates". Australian Journal of Soil Research. 43 (6): 767. doi:10.1071/SR04158.CS1 maint: multiple names: authors list (link)
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The idea that climate, vegetation, topography, parent material, and time control soils occurs in the writings of early naturalists. An explicit formulation was performed by Dokuchaev in 1898 in an obscure Russian journal unknown to western writers. He set down: soil = f(cl, o, p) tr
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The present paradigm views humus as a system of heteropolycondensates, largely produced by the soil microflora, in varying associations with clay (Anderson 1979). Because this conceptual model, and simulation models rooted within the concept, do not accommodate a large char component, a considerable change in conceptual understanding (a paradigm shift) appears imminent.
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